[Laser] Re : Cloud Bounce, Comm or Distance

Sat Sep 18 04:25:53 EDT 2010

Hi all.
To add some considerations about cloud bounce it is possible to precise 
these points;
The cloud bounce efficiency depends very strongly of the aerosol limit 
structures and the optical beams angular properties versus the clear 
air and the aerosol interface.
High average droplets size and density into high altitude stratus 
layers covering a large area are the best conditions.
The other very critical factor is the layer angle with the optical 
TX/RX beam axes that must be as flat as possible.
In these conditions the front scattering effect produce a real gain 
improvement that allows long range communication with very low power.
We got a -6 dB stable modulated signal with a 20 mW infrared laser (800 
nm) to 50 Km via a 1500 m ASL stratus layer.
The receiver used only a 22 cm spherical mirror (bathroom magnifier 
mirror :o)) and a K3PGP RX with a good PIN photodiode;
You can load  a ( 20 Mo) film at :

http://sd-1.archive-host.com/membres/videos/22679775843705539/F5ONY_cloud_bounce_theory.avi

F5ONY did this very simple experiment proving that the droplets at the 
lower part of a cloud can produce a very strong front scatter with a 
very great enhancement  to the receiver.
It is one of the strange properties of the Mie scattering effect.
73
Yves F1AVY
http://f1avyopto.wifeo.com

-----E-mail d'origine-----
De : Chris L <vocalion1928 at hotmail.com>
A : laser at mailman.qth.net
Envoyé le : Samedi, 18 Septembre 2010 1:53
Sujet : Re: [Laser] Cloud Bounce, Comm or Distance

Speaking for our group of Australian experimenters, I can definitely
state that all of these concepts have been considered, and most of 
these
ideas have actually been field tested over the last four decades of our
activity. The green and blue PhlatLight  LEDs actually have a higher
radiometric efficiency than the red, and the green LED has a much 
higher
photometric (eye response) efficiency, but these have far less
compatibility with the spectral photoresponse of silicon, which favours
longer visual wavelengths and the near-IR. The green LED's phosphor has
a much slower rise time than the red PhlatLight, and it has a much
broader half-power spectral bandwidth.

When one goes to digital, even for the transmission of speech, the
modulated tx and rx bandwidth must increase markedly. With Si P-I-N
photodiode receivers, this inevitably means a rapid trade-off of signal
to noise ratio against bandwidth, a redesign of the receiver's pre-amp
to suit that broader bandwidth, and the acceptance of far smaller tx
ranges. Avalanche photodiodes could provide part of the answer to the
speed problem, but they are around thirty (30) times more expensive 
than
the equivalent P-I-N photodiode, and they suffer from numerous noise
sources, some thermal, and some due to their avalanche signal
multiplication process, which is not noise free. The high
thermally-derived dark current of avalanche photodiodes makes it
difficult for them to equal photomultiplier sensitivity, even if
sufficiently high current gain at low levels is achieved to overcome
their thermal noise. The avalanche photodiodes have the advantage in
quantum efficiency, but this may not be sufficient in itself for them 
to
be a superior photodetector, overall, to a traditional photomultiplier 
tube.

An alternative is to use the blue-violet PhlatLight LED, in combination
with a receiver employing a surplus photomultiplier - a superb and
underestimated device, uniquely combining large sensitive area with
extremely high speed, and mostly suiting the detection of the shorter
visual wavelengths. However, in a direct line-of-sight link, the much
greater scattering, poorer atmospheric transmission and much higher
refractive scintillation of blue light renders this alternative
wavelength range unattractive, at least for atmospheric optical DX.

The exception to this objection occurs where scatter, cloud bounce or
non-line-of-sight ("NLOS") linking is contemplated. In this instance,
the much greater Rayleigh scattering potential of shorter wavelengths
(proportional to the inverse of the wavelength to the fourth power)
could actually become advantageous, especially for molecular scattering
in a cloudless sky. With most scatter links of this type, a fast and
relatively large area photodetector (ideally, again, a photomultiplier)
is necessary to intercept the large image dimensions of an extended
scattering field in the receiver optic's focal plane. In this NLOS
instance, the ideal optics would be Fresnel lenses or parabolic
searchlight reflectors of very large aperture, perhaps in excess of a
metre in diameter, to maximise tx optical gain and to maximise the
scattered photon collection area for rx. The area of the sky 
illuminated
should be minimised via the production of the narrowest possible tx
beam, so that that the scatter field can be imaged on a photodetection
area of minimum dimensions, for two reasons:

(1) A concentration of high flux in the smallest possible part of the
sky will maximise the modulated scatter illumination against ambient
(background) illumination.

(2) Photodetector sensitive area is always directly proportional to the
photodetector's thermal noise, so it is always desirable that the
photodetector dimensions should be matched, as closely as possible, to
the image dimensions of the scatter field. The image size of the sky
scatter field can be reduced in the receiver's focal plane by reducing
the receiver optic's focal length as far as possible, and/or by the
spreading of the scatter field in altitude, but not in azimuth. The
azimuth of a given scatter field (if not for cloud bounce) will always
maximise when two NLOS scatter stations are pointed precisely in each
other's direction, with a minimal path distance between the two.
However, if the tx beam spreads in the vertical direction only, and the
azimuth of the beam remains the same, the shaft of light seen at the
distant station will not occupy a larger sky area. It may be desirable
to have independently controllable vertical and lateral beam 
divergence,
and to mask the receiver's sensitive area to admit only the part of the
sky including an image of the transmit beam's scatter field, which will
usually be seen as a shaft extending vertically from the horizon in the
direction of the transmitter. In other words, the mask will admit light
 from a vertical slot, matching the image of the scattered vertical 
light
shaft.

By following these fundamental optical and practical principles, we
contend that it should be possible to dispense with wspr, WSJT and 
other
sub-noise detection systems on such a NLOS link. The recovered
sig./noise should then be higher by many tens of dB, and speech linking
may be quite possible by using large high-gain optical systems, 
accurate
tracking, and high intensity sources such as blue PhlatLight LEDs. In
recent times, another group of hams used WSJT to bridge about 300 km
NLOS, by intentionally spreading a transmit beam to some 5 degrees of
divergence from the tx, and by intercepting only a tiny part of the
scattering field. Sixty (60) Luxeon LEDs behind 60 page magnifiers were
used, with commercial Lumileds  catadioptic "torch" reflectors as
secondary reflectors on each Luxeon. We would contend that these optics
are excessively cumbersome, needlessly expensive, and very poorly
conceived. Intentional spreading of a tx beam for scatter propagation 
in
the presence of ambient is a fundamentally flawed concept. Added to 
that
problem, the tiny avalanche photodiodes used for reception had
inappropriately small collection areas for the NLOS scatter system. If
the emitters, detectors and optical systems had been better designed 
and
matched, no recourse to WSJT would have been necessary.

The usage of WSJT or similar sub-noise "digital" systems, capable only
of transmitting call signs over periods of 10 minutes or more, have too
often been used in circumvention of sound optical design. By the usage
of WSJT, many optical system failures tend to have been dressed up as
successes. I don't think that anyone would argue that progress lies in
that direction.

In other words, there is AMPLE scope for improvement, here!

Best wishes,

Chris Long VK3AML.

David Learmonth VK3QM.

============================================

> Date: Thu, 16 Sep 2010 20:48:36 -0400
> From: n5gui at cox.net
> To: mike1 at mgte.com; laser at mailman.qth.net
> Subject: Re: [Laser] Cloud Bounce, Comm or Distance
>
>
> ---- Mike <mikecouture at bellsouth.net> wrote:
> >
> > Do we have any interest in cloud bounce for either communications or
> > distance measuring using low to medium power lasers or LED's?
> >
>
>
> Your question got me to thinking ( which is usually dangerous ) about 
a couple
of things.
>
> The first is perhaps a subset of cloud bounce.  The classic idea of 
cloud
bounce, to me at least, is that you see a cloud in the sky, you point 
your light
beam at it, and then someone sees your "spot" and decides to reply.  I 
have
frequently seen an advertising searchlight hit clouds so this does not 
take a
lot of imagination.
>
> There are atmospheric conditions other than puffy cotton ball clouds 
that
might be used.  One such condition causes rainbows or fogbows.  I am 
thinking
more of thin ice clouds.  Often in winter there are thin ice crystal 
clouds,
usually observed with sun dogs or halos, or their equivalents from 
bright
moonlight.  These ice crystal clouds are hard to see directly.  The ice 
crystals
form different shaped prisms or flat hexagonal plates which reflect and 
/ or
refract light at predictable, but narrow angles.  ( That was a hint for 
the
curious to investigate the atmospheric physics involved. )  A beam of 
light,
whether from the sun, moon, or optical communication equipment, comes 
away in a
cone defined by that angle.
>
> In order to use the reflections/refractions for communications you 
would need
considerably more skill, or perhaps extraordinary luck, than the more 
typical
cloud bounce.  I would not suggest anyone start on a project to use ice 
crystal
reflections instead of, or before the puffy cloud bounce.  But maybe it 
is the
next step ( or the next challenge after ).
>
> The second thing was listening practice for cloud bounce, for either 
the puffy
cloud version or the ice crystal reflection/refraction version.  In the 
densely
populated areas, streetlights and other manmade lights pulsing at 60 
Hertz and
various harmonics, should be detectable on every cloud overhead.  I 
live in
Kansas, so there are places with some pretty dark skys, so I should be 
able to
find places where only limited sites can illuminate the clouds, puffy 
or icy.
Predicting and then confirming the cloud bounce, seems to be a useful 
practice
before trying to communicate.  I am not currently working on such a 
project, but
I thought I would pass the idea along in case someone else could use it.
>
>
> James
>  n5gui
>
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